Boreal streams receive a significant amount of terrestrially derived carbon (C) from surrounding soils and vegetation. Its decomposition and processing are primarily driven by microbial decomposers, namely fungi and bacteria. The rate of breakdown is determined by the availability and recalcitrance of two key C sources: leaf litter and dissolved organic carbon (DOC). Previous research indicates an antagonistic interaction between leaf litter and DOC decomposition, but the underlying dynamics have not been fully understood. This study investigates how DOC flux influences microbial decomposition of leaf litter, examining changes in litter mass loss and microbial respiration over time in response to C availability. Leaf litter of various recalcitrance (spruce, birch, and alder, ordered by decreasing recalcitrance) was exposed to a variety of DOC flux rates in a set of 12 experimental channels within the Krycklan Catchment Study area in northern Sweden. Each channel received a distinct discharge sourced from a natural stream, creating a gradient of water flow rates across the experimental setup, with discharges ranging from 0.04 to 0.88 L/s. We hypothesize that increased DOC flux, together with differences in litter recalcitrance among litter species and changes in recalcitrance over time, drives a microbial switch in resource use, leading to the preferential use of DOC as the main C source. This process could influence C cycling and ecosystem functioning in boreal streams, especially under changing climate and land use conditions.